During the past year, over 450 peptides have been produced by a PRI contract facility and within the Branch itself. These peptides have supported research projects in nearly all Laboratories of the NIAID. Synthetic peptides have been used to study the interaction of peptides with MHC class I and class II molecules. Such peptides have been used to define cytotoxic T lymphocytes or helper T cell epitopes of proteins from chlamydia, plasmodia, HIV, and influenza A and B as well as from heat shock proteins and myelin basic protein. Synthetic peptides have been used to prepare antisera against proteins originally described by gene cloning or protein sequencing methodologies. Such antisera have been prepared against proteins from the following sources: Giardia lamblia, respiratory syncytial virus, DRalpha and Beta chains, vaccinia, Friend virus, HIV, respiratory acquired infectious anemia virus, Leishmania proteins, IgE receptor, and Dengue virus. Peptides have also been used to define the functional regions of cytochrome b558, Leishmania proteins and a Herpes simplex virus leucine zipper containing protein. Within the Branch, synthetic peptides were used to define peptidic epitopes from HTLV-I and - II proteins that were reactive with patient's sera. Multiple epitopes were defined from env, gag and pol encoded proteins, and one gp46 derived peptide allowed categorical distinction between HTLV-I and II infections by ELISA. The SCL gene product, which is thought to be fundamentally important for hemopoietic differentiation, was described using an anti-peptide serum.